Field of the Invention
The present invention relates to nuclear magnetic resonance (NMR) apparatus for scanning and imaging, and more particularly to NMR apparatus which include a primary magnetic field produced by an assembly of low energy permanent magnetic elements.
NMR apparatus are used in a wide variety of analytical and diagnostic applications. The primary components of an NMR apparatus used for imaging biological tissue, and more particularly whole body imaging of biological tissue, include a primary magnetic field, a source of radio-frequency energy, and a detector. The object to be analyzed is positioned within the primary magnetic field and is simultaneously subjected to the radio-frequency energy. The radio-frequency energy is absorbed by the object to be analyzed thereby elevating the energy level of the atomic neuclei of the object. After the irradiation of the atomic neuclei are completed, the object is allowed to return to its initial lower energy state by re-radiating the absorbed energy. That re-radiated energy is measured by the detector. Standard well-known NMR techniques are then used to analyze the detected signals to provide an image.
In order to effectively image a subject such as an entire human body, the magnet providing the primary magnetic field must be constructed of sufficient size to accomodate the human body therein. In addition, the primary magnetic field must be of sufficient strength and substantially uniform over a relatively large area in order to construct images from the re-radiated energy.
In the past, NMR apparatus have employed air core electromagnets to produce the primary magnetic field. In order to obtain stronger magnetic fields, air core super-conducting electromagnets were later used. These super-conducting electromagnets can be operated only at approximately -269.degree. C. The systems necessary to produce and maintain such temperatures are expensive, difficult to manufacture, and difficult to maintain. Cooling of the super-conducting electromagnets is accomplished by submerging coils in a liquid helium container which is thermally isolated from room temperature by a combination of vacuum, super-insulation material, and a liquid nitrogen cooled shield. Those substances boil off and must constantly be replenished. Costs of $100,000.00 a year in liquid helium and nitrogen to keep the electromagnet in operation are not uncommon.
Furthermore, the magnetic lines of flux generated by air core electromagnets cover an immense area and, because interference with these lines of magnetic flux can cause inappropriate NMR responses, elaborate sanctuaries isolating the NMR apparatus from the remainder of the hospital environment must be constructed. While NMR apparatus using super-conducting electromagnets have provided excellent results, the complexity and costs associated with the vacuum and cooling systems as well as the large amount of isolated space required have limited the wide-spread application of such apparatus.
In U.S. Patent Application Ser. No. 650,558 filed Sept. 13, 1984, and assigned to the same assignee as the present invention, now abandoned it was recognized that a permanent magnet formed of a combination of relatively low energy magnetic material and relatively high energy magnetic material could be used to provide a primary magnetic field of sufficient strength per unit volume. Prior to that invention, it had been thought impractical to employ a permanent magnet for producing the primary magnetic field.